1. Field of the Invention
This invention relates to drill bits.
2. Description of the Prior Art
In an ordinary drill, there is a chisel provided in the neighborhood of the axis of rotation. While the chisel is a peculiar feature of a drill or the like, it is also a drawback, that is, it gives rise to high thrust load and torque load. Particularly, high thrust load often causes breakage of the drill or cause damage to the cutting edge, and various methods have been contrived to solve this problem. More specifically, the prior art solid type drill has cutting edges and flanks formed at the tip portion and has margins on the body, and it also has a chisel edge formed at dead center. When a workpiece is drilled using this drill, the cutting edges are fed in while cutting the workpiece. At this time, the chisel edge is forced into workpiece rather than entering the workpiece by cutting and experiences excessive thrust load. Therefore, it suffers pronounced wear or damage, which curtails its service life, and is prone to breakage.
Another drawback of the chisel edge is that the very high resistance offered to it causes "grinding motion" of the drill with its opposite ends alternately constituting an axis of rotation in the initial stage of drilling operation. The "grinding motion" leads to "vibration", and once this occurs, the workpiece can no longer be cut and the tool is broken.
In order to solve the above problems, a drill having a thinned chisel, i.e., a drill with a reduced length of chisel edge, has been proposed. However, reducing the width of the chisel reduces its mechanical strength. Besides, the thinned portion is subject to greater wear.
To solve all the problems due to the presence of the chisel, there has been proposed a drill which is free from any chisel, more particularly free from any cutting edge in a zone where there is a chisel in the usual drill (as disclosed in Japanese Patent Application Public Disclosure No. Sho 56(1981)-52110). In this drill, a gap or cavity is formed at the center of the point, and material of the work that remains in a cylindrical form in the gap while the workpiece is being drilled is twisted apart by cutting resistance. By virtue of the freedom from a chisel of this drill, it is possible to alleviate the thrust load and suppress grinding motion. However, material of the work in the neighborhood of the axis of rotation of the tool is not cut out but plucked out by shearing force. This is undesired from the standpoint of machining operation.